When a person is accused of a crime, many different types of evidence may used by law enforcement to find the perpetrator or be introduced by the prosecution at trial in order to convince the judge or jury of the defendant's guilt. Evidence that may be introduced at trial may include documentary, tangible, or biological evidence, among other types. Examples of evidence that may be considered biological evidence include blood, saliva, and hair, as well as skin, semen, and nail clippings.
When a crime is committed, it is extremely rare that no biological evidence is left behind by the perpetrator. People leave trace evidence of their presence everywhere that they visit as a rule. Hair, for instance, routinely falls out in order to make room for more to grow. Skin also naturally exfoliates, or flakes off, in order to make room for the newer skin cells.
Despite efforts by many criminals to prevent leaving any evidence of who they are, the vast majority of the time, some evidence is left behind. This is particularly true in the case of a violent crime. When a struggle takes place, the chances of the perpetrator leaving behind biological evidence increases significantly. This evidence may be found at the crime scene or on the victim himself or herself.
Due to the rapid advances in science during the 20th century, analysis of biological evidence often leads to the identity of a perpetrator. When biological evidence is located at a crime scene or on a victim, it is collected and sent to a laboratory for analysis. Depending on the type of evidence, the analysis may be able to tell law enforcement a variety of things, such as whether the perpetrator is male or female, an approximate age, and what color hair he or she has.
Along with providing clues during the investigation stage of a crime, biological evidence may be used to convict the defendant at trail. For years, blood evidence was used to rule out a defendant if the blood type found was not the same as the defendant's. When the blood types were the same, it was used as evidence by the prosecution, but was rarely considered determinative. Other biological evidence has also historically been used to help determine the identity of a criminal or to rule out a potential suspect.
With the advances in DNA testing, blood evidence can now be used to compare blood found at a crime scene with a potential suspect's blood with an accuracy rate of better than 99%. DNA testing is considered to be so accurate that, along with helping to convict defendants, it has also helped to free many wrongly-convicted people. Within the United States, many people have been released from prison, and their convictions overturned, due to a DNA analysis of blood that was unavailable at the time of conviction, which rules them out as the possible perpetrator of the crime.